Paper friction material and method of manufacturing the same

ABSTRACT

This invention relates to a paper friction material and a method of manufacturing the same, wherein the paper friction material is configured such that dispersibility and bondability are increased between a filler and a matrix. The paper friction material which is suitable for use in a vehicle, includes a friction base including pulp and a coating layer formed on the friction base using a mixture of latex and a functional material. The OH reactive group of the pulp and the aromatic ring of the latex are hydrogen-bonded so that the friction base and the coating layer are coupled with each other.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims under 35 U.S.C.§ 119(a) the benefit ofKorean Patent Application No. 10-2016-0110000, filed Aug. 29, 2016, theentire contents of which are incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a paper friction material and a methodfor the manufacture thereof. Provided herein is a paper frictionmaterial and a method of manufacturing the same in which the paperfriction material is configured such that dispersibility and bondabilityare increased between a filler and a matrix.

Description of the Related Art

In order to be useful for an automatic transmission clutch, paper isrequired to have superior physical properties, including friction andwear properties, durability, impregnation porosity and the like. Inparticular, since an automatic transmission clutch operates in the stateof being immersed in transmission oil, it has to possess superiorhigh-temperature friction properties, wear resistance and durabilityeven in the state of being impregnated with oil.

Examples of wet clutch materials that have been used to date includepaper, sintered Cu or resin, graphite, etc. Paper is mainly used as afriction material for a vehicle. Paper is composed mainly of pulp andincludes an inorganic filler and a thermosetting resin (e.g., phenol,epoxy, melamine resin). The kind and amount of inorganic filler areknown to have aninfluence on the friction properties of paper.

Particularly, in order to increase NVH (Noise, Vibration, Harshness)reduction performance, as is required of an automatic transmission, thefriction surface should be uniform, microsized particles for absorbingoil that is uniformly applied thereon. The microsized particles shouldbe prevented from being separated from the friction material duringusage.

To this end, a functional coating layer is conventionally formed on thesurface of a friction material. The coating layer is formed by preparinga coating solution and applying the coating solution on the surface ofthe friction material. As such, applying the coating solution on thesurface of the friction material may be performed using a bar-typecoater or a wood or rubber roller. However, this coating process isproblematic because it is difficult to apply the coating solution to auniform thickness, and thus, the functional material and the filler,which are contained in the applied coating solution, are not uniformlydistributed.

Details set forth as the background art are provided for the purpose ofbetter understanding the background of the invention, but are not to betaken as an admission that the described details correspond to theconventional technology already known to those skilled in the art.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems encountered in the related art, and the present inventionis intended to provide a paper friction material and a method ofmanufacturing the same, wherein functional fine particles are uniformlyapplied on the surface of a friction material and adhesion is enhancedso that the functional particles are prevented from being separated fromthe surface of the friction material during usage of the frictionmaterial.

An embodiment of the present invention provides a paper frictionmaterial for a vehicle, comprising: a friction base including pulp, anda coating layer formed by applying a mixture comprising latex and afunctional material on the friction base, wherein an OH reactive groupof the pulp and an aromatic ring of the latex are hydrogen-bonded sothat the friction base and the coating layer are coupled with eachother.

The friction base may include from about 35 wt % to about 45 wt % (e.g.,about 35 wt %, 36, 37, 38, 39, 40, 41, 42, 43, 44, or about 45 wt %) ofa matrix including pulp, from about 5 wt % to about 15 wt % (e.g., about5 wt %, 6, 7, 8, 9, 10, 11, 12, 13, 14, or about 15 wt %) of areinforcement, from about 15 wt % to about 25 wt % (e.g., about 15 wt %,16, 17, 18, 19, 20, 21, 22, 23, 24, or about 25 wt %) of a frictionmodifier, and the remainder of a filler.

The reinforcement may include an aramid fiber, the friction modifier mayinclude coke, and the filler may include at least one of diatomite andsilicon nitride.

The coating layer may be formed through spray coating using a coatingsolution comprising the latex and the functional material, which aremixed.

The coating solution may include from about 50 wt % to about 60 wt %(e.g., about 50 wt %, 51, 52, 53, 54, 55, 56, 57, 58, 59, or about 60 wt%) of water, from about 22 wt % to about 28 wt % (e.g., about 22 wt %,23, 24, 25, 26, 27, or about 28 wt %) of the latex, from about 15 wt %to about 25 wt % (e.g., about 15 wt %, 16, 17, 18, 19, 20, 21, 22, 23,24, or about 25 wt %) of a filler, and about 8 wt % or less (e.g., about8 wt %, 7, 6, 5, 4, 3, about 2 wt % or less) of the functional material.

The filler may include diatomite and the functional material may includeat least one of graphite and molybdenum sulfide (MoS₂).

Another embodiment of the present invention provides a method ofmanufacturing a paper friction material for a vehicle, comprising:preparing a friction base including pulp, preparing a coating solutioncomprising latex and a functional material, which are mixed, coating thefriction base with the coating solution using a spray, thus forming, onthe surface of the friction base, a coating layer configured such thatan OH reactive group of the pulp of the friction base and an aromaticring of the latex of the coating solution are hydrogen-bonded, anddrying the friction base having the coating layer formed thereon.

In this method, preparing the friction base may include: mixing fromabout 35 wt % to about 45 wt % (e.g., about 35 wt %, 36, 37, 38, 39, 40,41, 42, 43, 44, or 45 wt %) of a matrix including pulp, from about 5 wt% to about 15 wt % (e.g., about 5 wt %, 6, 7, 8, 9, 10, 11, 12, 13, 14,or about 15 wt %) of a reinforcement, from about 15 wt % to about 25 wt% (e.g., about 15 wt %, 16, 17, 18, 19, 20, 21, 22, 23, 24, or about 25wt %) of a friction modifier, and the remainder of a filler to give amixture which is then subjected to dehydration pressing to form a sheet,thus manufacturing a friction base, and drying the friction base.

Also, preparing the coating solution may include mixing from about 50 wt% to about 60 wt % (e.g., about 50 wt %, 51, 52, 53, 54, 55, 56, 57, 58,59, or about 60 wt %) of water, from about 22 wt % to about 28 wt %(e.g., about 22 wt %, 23, 24, 25, 26, 27, or about 28 wt %) of thelatex, from about 15 wt % to about 25 wt % (e.g., about 15 wt %, 16, 17,18, 19, 20, 21, 22, 23, 24, or about 25 wt %) of a filler and from about8 wt % or less (e.g., about 8 wt %, 7, 6, 5, 4, 3, about 2 wt % or less)of the functional material.

Also, preparing the coating solution may include: (1) mixing the waterwith the filler and the functional material, thus obtaining a coatingsolution, (2) stirring the resulting coating solution, (3) mixing thestirred coating solution with the latex, and (4) stirring the resultingmixed coating solution.

The method may further include impregnating the friction base with aphenol resin, squeezing the friction base impregnated with the phenolresin, and performing heat treatment for hardening the phenol resinincorporated into the friction base. After the drying, the friction basehas the coating layer formed thereon.

According to embodiments of the present invention, a coating layer canbe uniformly formed on the surface of a friction material through aspray-coating process using a coating solution comprising latex andfunctional particles, thereby increasing the NVH reduction performanceof the friction material.

Since the coating layer is formed using latex, adhesion of the coatinglayer is enhanced, whereby the functional particles can be preventedfrom being separated from the friction material during usage of thefriction material.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill be more clearly understood from the following detailed descriptiontaken in conjunction with the accompanying drawings.

FIG. 1 is a flowchart showing a process of manufacturing a paperfriction material according to an embodiment of the present invention.

FIG. 2 is a graph showing the results of shear strength of ComparativeExample and Example according to the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a detailed description will be given of embodiments of thepresent invention with reference to the appended drawings. However, thepresent invention is not limited to the following embodiments, which maybe changed to have various forms. These embodiments are provided tocomplete the disclosure of the present invention and to fully describethe present invention to those skilled in the art.

According to an embodiment of the present invention, a paper frictionmaterial (hereinafter, referred to as a “friction material”) includes afriction base including pulp and a coating layer formed by applying amixture comprising latex and a functional material on the friction base.As such, the OH reactive group of the pulp and the aromatic ring of thelatex are hydrogen-bonded, whereby the friction base and the coatinglayer are coupled with each other.

The friction base constitutes most of the paper friction material inorder to obtain the desired compression resistance, mechanical strengthand heat resistance. The friction base can be in an amount of about 95wt % based on the total weight of the paper friction material.

The friction base can be composed of, based on wt % (hereinafter, %indicates wt % unless otherwise stated), from about 35% to about 45%(e.g., about 35 wt %, 36, 37, 38, 39, 40, 41, 42, 43, 44, or about 45 wt%) of a matrix including pulp, from about 5% to about 15% (e.g., about 5wt %, 6, 7, 8, 9, 10, 11, 12, 13, 14, or about 15 wt %) of areinforcement, from about 15% to 25% (e.g., about 15 wt %, 16, 17, 18,19, 20, 21, 22, 23, 24, or about 25 wt %) of a friction modifier, andthe remainder of a filler.

The matrix, which constitutes the friction base, is a helical-typelinter pulp. The helical-type linter pulp is characterized by softnessand thus frictional noise is reduced. The bonding angle or internalrotation angle within the molecular chain is somewhat consistentlyformed, whereby high resistance to continuous fatigue and highresilience may result. Also, oil, which is absorbed into and stored inthe friction material, may be instantly released like a sponge, and thusheat generated upon slipping is rapidly dissipated and the durability ofthe friction material may increase.

Linter pulp is used as the matrix of the friction base such that ahydrogen bonding layer may be formed using the O—H pulp structurecontained in the linter pulp.

The matrix is preferably used in an amount of about 35 to 45% (e.g.,about 35%, 36, 37, 38, 39, 40, 41, 42, 43, 44, or about 45%) in order tomaintain the aforementioned physical properties.

The reinforcement is added to impart tensile strength, toughness, heatresistance and elastic modulus to the friction material. Thereinforcement may be composed of various components depending on end useof the friction material. The amount of the reinforcement is limited toa range of about 5% to 15% (e.g., about 5 wt %, 6, 7, 8, 9, 10, 11, 12,13, 14, or about 15 wt %). Hence, the physical properties that areanticipated in the friction base due to the addition of thereinforcement may be retained, and furthermore, the physical propertiesthat are anticipated from the inclusion of the reinforcement may bemaintained.

In this embodiment, a reinforced fiber, for example, an aramid fiber,can be used as the reinforcement. Thus, high tensile strength,toughness, heat resistance and elastic modulus are imparted to thefriction material, and frictional noise may be reduced due to theproperty of softness.

The friction modifier can be added to increase the friction propertiesof the friction material. In the present embodiment, coke is used.

The amount of the friction modifier is limited to a range of about 15%to 25% (e.g., about 15 wt %, 16, 17, 18, 19, 20, 21, 22, 23, 24, orabout 25 wt %). And thus noise and vibration of the friction materialare preferably reduced to a desired level by the addition of thefriction modifier.

The friction base is composed of the matrix, the reinforcement and thefriction modifier, which are mixed in desired amounts, with theremainder of the filler. The filler, which is used in the remainingamount, can be made of a porous material to minimize inter-surfaceslipping and achieve complete contact. The filler preferably includes atleast one of diatomite and silicon nitride. Diatomite and siliconnitride have pores and a particle size on a micron scale. When pores forabsorbing oil are distributed throughout the matrix, an oil lubricantfilm may be favorably formed. When diatomite and silicon nitride areused as the filler, noise and vibration reduction performance may beincreased.

The coating layer is formed on the surface of the friction base. Thecoating solution comprising latex and a functional material, which aremixed together, can be applied through spray coating.

In some embodiments, the coating solution is composed of about 50% toabout 60% (e.g., about 50 wt %, 51, 52, 53, 54, 55, 56, 57, 58, 59, orabout 60 wt %) of water, about 22% to about 28% (e.g., about 22 wt %,23, 24, 25, 26, 27, or about 28 wt %)of latex, about 15% to about 25%(e.g., about 15 wt %, 16, 17, 18, 19, 20, 21, 22, 23, 24, or about 25 wt%) of a filler, and about 8% or less (e.g., about 8 wt %, 7, 6, 5, 4, 3,2, 1 wt % or less) of a functional material. The filler can includediatomite, and the functional material preferably includes at least oneof graphite and molybdenum sulfide (MoS₂).

If the amount of water is less than 50%, the coating nozzle may clog inthe spray-coating process and the coating process may be non-uniformlyperformed. On the other hand, if the amount of water exceeds 60%,bondability may decrease and the surface adsorbability of the frictionmaterial may become problematic.

The latex is used to firmly attach the filler and the functionalmaterial to the surface of the friction base. A strong bond ismaintained because of the bonding of the latex alone as well as thehydrogen bonding between the OH reactive group of the linter pulp andthe aromatic ring of the latex can increase bonding energy. Thus, theseinteractions can provide the bonding strength of a monolayer structurebetween the friction base and the coating layer.

If the amount of latex is less than 22%, the filler and the functionalmaterial are not sufficiently bonded to the surface of the frictionmaterial by the components of the latex, and thus, in the squeezingprocess after the impregnation, the filler and the functional materialmay be stripped off. On the other hand, if the amount thereof exceeds28%, the pores of the filler and the friction material may be cloggedand oil flow may decrease, undesirably causing noise problems.

When the amount of diatomite used as the filler is 15% or more, it isuniformly distributed on the corresponding surface to thus exhibitsuperior noise-reduction performance. If the amount thereof exceeds 25%,mineral particles are excessively sprayed in the spray-coating process,undesirably causing reliability problems.

As the functional material, graphite and molybdenum sulfide (MoS₂) aresolid lubricants, and the amount thereof is preferably limited to 8% orless in order to improve lubricating properties.

A method of manufacturing the paper friction material is described belowwith reference to the appended drawing.

FIG. 1 is a flowchart showing the process of manufacturing the paperfriction material according to an embodiment of the present invention.

As shown in FIG. 1, the method of manufacturing the paper frictionmaterial according to an embodiment of the present invention includespreparing a friction base (S100), preparing a coating solution (S200),forming a coating layer (S300), drying the friction base (S400),impregnating the friction base with a phenol resin (S500), squeezing thefriction base (S600), and performing heat treatment (S700).

When preparing the friction base (S100), the friction base is providedin the form of a single layer.

The friction base is prepared by combining a matrix, a reinforcement, afriction modifier and a filler in the aforementioned amounts and thenmixing the resulting combination with water using a mixer so as toachieve uniform distribution (S110).

The mixture thus obtained is placed in a sheet former and thendehydration-pressed, whereby the mixture is uniformly left behind on thewire mesh of the sheet former (S120). The dehydration press is operatedat 2500 to 3000 psi for 8 to 12 sec so that up to approximately 90% ofthe water is removed.

Next, the mixture in sheet form is separated from the wire mesh and thendried at from about 140° C. to 160° C. (e.g., about 140° C., 141, 142,143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156,157, 158, 159, or 160° C.) for about 8 to 12 min (e.g., about 8 min, 9,10, 11, or 12 min), thus preparing a friction base (S130). The frictionbase thus prepared has a water content of less than 5%.

After preparation of the friction base, a coating solution, which is tobe applied on the friction base, is prepared (S200).

The coating solution is prepared by mixing water, latex, a filler and afunctional material in the aforementioned amounts and then stirringthem. As such, preparing the coating solution is performed throughmultiple procedures so that other materials are prevented fromcoagulating due to the aggregation force of latex to thus prevent theclogging of the nozzle of a spray gun in the spray-coating process.

Specifically, water is mixed with a filler and a functional material inpredetermined amounts (primary mixing; S210). Next, the coating solutionobtained in the primary mixing process is sufficiently stirred (primarystirring; S220).

When water is sufficiently mixed with the filler and the functionalmaterial in this way, the coating solution, which is primarily stirred,is mixed with latex in a predetermined amount (secondary mixing; S230).Next, the secondarily mixed coating solution is sufficiently stirred toprepare a coating solution (secondary stirring; S240).

When the friction base and the coating solution are prepared in thisway, the coating solution is applied on the friction base using a spray(S300). The coating solution is sprayed on the friction base, whereby acoating layer is formed on the surface of the friction base while the OHreactive group of the linter pulp of the friction base and the aromaticring of the latex of the coating solution are hydrogen-bonded.

After spray coating with the coating solution, water and latex, actingas the binder in the coating solution, are dried to increase thebondability of the coating layer formed on the surface of the frictionbase (S400). Here, the loss of the coating solution is prevented using anon-contact dryer such as a hot air dryer, instead of a contact-typedryer.

After formation of the coating layer on the surface of the frictionbase, the friction base having the coating layer formed thereon isimpregnated with a phenol resin to increase the tensile strength andshear strength of the friction material and improve heat resistance andwear resistance (S500).

When the friction base having the coating layer formed thereon issufficiently impregnated with the phenol resin, the phenolresin-impregnated friction base is squeezed by passing it through rollsat a predetermined interval, so that the phenol resin is uniformlydistributed in the friction base having the coating layer formed thereon(S600).

After squeezing, heat treatment is performed to harden the phenol resinincorporated into the friction base, thereby completing the fabricationof the friction material (S700). Here, heat treatment is performed atabout 170° C. to about 180° C. (e.g., about 170° C., 171, 172, 173, 174,175, 176, 177, 178, 179, or about 180° C.) for about 3 to about 5 min(e.g., about 3 min, 4 min, or about 5 min).

Below, the present invention is described through the followingComparative Example and Example.

In the following tests, the Example of the present invention is afriction material (comprising two layers) configured such that a coatinglayer is formed on a friction base using a spray-coating process, andComparative Example is a friction material (comprising one layer) havingno coating layer.

Testing for measuring shear strength was performed. The results areshown in FIG. 2. Shear strength in Comparative Example and Example wasmeasured according to ASTM D1002 (Lap shear strength test).

FIG. 2 is a graph showing the results of shear strength of Example ofthe present invention and Comparative Example.

As shown in FIG. 2, in the case of Example, internal bonding strengthequal to or greater than that of Comparative Example was exhibited dueto the complete bonding of the latex binder.

In addition, the friction stability of the friction materials ofComparative Example (Comparative Example 1) and Example was measured.The results are shown in Table 1 below.

To evaluate noise and vibration properties, tests were performed usingan international standards-compliant LVFA, and friction stability wasevaluated based on the obtained dμ/dv values. Here, dμ/dv is the slopeof coefficient of friction relative to speed. When dμ/dv has a positive(+) value, stick-slip does not occur and noise and vibration aresignificantly reduced. On the other hand, when dμ/dv has a negative (−)value, the likelihood of generating stick-slip, which causes vibration,may increase, and thus noise and vibration are not reduced. That is,when the dμ/dv value is greater than 0, noise and vibration may be morefavorably reduced.

TABLE 1 Comp. Example 1 Example dμ/dv dμ/dv dμ/dv dμ/dv (50 to (100 to(50 to (100 to Test 100 km) 50 km) 100 km) 50 km) LVFA 40° C. 2.0 0.59.9 1.2 test 80° C. −2.7 −2.7 10.6 4.4 (0 hr) 120° C.  −6.7 −3.0 19.414.8 LVFA 40° C. — — 9.8 6.4 test 80° C. — — 9.0 2.5 (24 hr) 120° C.  —— 11.8 3.8 LVFA 40° C. — — 11.4 5.9 test 80° C. — — 13.5 6.2 (48 hr)120° C.  18.0 6.3

As is apparent from the results of Table 1, the dμ/dv value of Examplehad a positive value, unlike Comparative Example 1, thus ensuringfriction stability.

Also, in order to evaluate the effects of latex used for the coatingsolution, the friction stability of the friction materials of Example ofthe present invention and Comparative Example 2 using a phenol resininstead of the latex of the coating solution was evaluated. The resultsare shown in Table 2 below.

TABLE 2 Comp. Example 2 Example dμ/dv dμ/dv dμ/dv dμ/dv (50 to (100 to(50 to (100 to Test 100 km) 50 km) 100 km) 50 km) LVFA 40° C. −2.2 −3.79.9 1.2 test 80° C. −1.9 −2.9 10.6 4.4 (0 hr) 120° C.  −9.6 −4.1 19.414.8 LVFA 40° C. — — 9.8 6.4 test 80° C. — — 9.0 2.5 (24 hr) 120° C.  —— 11.8 3.8 LVFA 40° C. — — 11.4 5.9 test 80° C. — — 13.5 6.2 (48 hr)120° C.  — — 18.0 6.3

As is apparent from Table 2, the dμ/dv value of Example had a positivevalue, unlike Comparative Example 2, thus ensuring friction stability.

When the phenol resin was used, stability with the filler was decreasedand thus the filler was not uniformly applied upon spray coating. Due tothis problem, the friction surface is not relatively uniform, and theoil film is not uniformly formed upon a continuous slip test, thusdeteriorating NVH reduction performance.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes with reference to the appendeddrawings, those skilled in the art will appreciate that variousmodifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

What is claimed is:
 1. A paper friction material for a vehicle,comprising: a friction base including pulp; and a coating layer formedby applying a mixture comprising latex and a functional material on thefriction base, wherein an OH reactive group of the pulp and an aromaticring of the latex are hydrogen-bonded so that the friction base and thecoating layer are coupled with each other.
 2. The paper frictionmaterial of claim 1, wherein the friction base comprises from about 35wt % to about 45 wt % of a matrix including pulp, from about 5 wt % toabout 15 wt % of a reinforcement, from about 15 wt % to about 25 wt % ofa friction modifier, and a remainder of a filler.
 3. The paper frictionmaterial of claim 2, wherein the reinforcement comprises an aramidfiber, the friction modifier comprises coke, and the filler comprises atleast one of diatomite and silicon nitride.
 4. The paper frictionmaterial of claim 1, wherein the coating layer is formed through spraycoating using a coating solution comprising the latex and the functionalmaterial, which are mixed.
 5. The paper friction material of claim 4,wherein the coating solution comprises from about 50 wt % to about 60 wt% of water, from about 22 wt % to about 28 wt % of the latex, from about15 wt % to about 25 wt % of a filler, and from about 8 wt % or less ofthe functional material.
 6. The paper friction material of claim 5,wherein the filler comprises diatomite and the functional materialincludes at least one of graphite and molybdenum sulfide (MoS₂).
 7. Amethod of manufacturing a paper friction material for a vehicle,comprising: preparing a friction base including pulp; preparing acoating solution comprising latex and a functional material, which aremixed; coating the friction base with the coating solution using aspray, thus forming, on a surface of the friction base, a coating layerconfigured such that an OH reactive group of the pulp of the frictionbase and an aromatic ring of the latex of the coating solution arehydrogen-bonded; and drying the friction base having the coating layerformed thereon.
 8. The method of claim 7, wherein the preparing thefriction base comprises: mixing from about 35 wt % to about 45 wt % of amatrix including pulp, from about 5 wt % to about 15 wt % of areinforcement, from about 15 wt % to about 25 wt % of a frictionmodifier, and a remainder of a filler to give a mixture which is thensubjected to dehydration pressing to form a sheet, thus manufacturing afriction base; and drying the friction base.
 9. The method of claim 7,wherein the preparing the coating solution comprises mixing from about50 wt % to about 60 wt % of water, from about 22 wt % to about 28 wt %of the latex, from about 15 wt % to about 25 wt % of a filler, and fromabout 8 wt % or less of the functional material.
 10. The method of claim9, wherein the preparing the coating solution comprises: primarilymixing the water with the filler and the functional material, thusobtaining a coating solution; primarily stirring the coating solution;secondarily mixing the primarily stirred coating solution with thelatex; and secondarily stirring the secondarily mixed coating solution.11. The method of claim 7, further comprising, after the drying thefriction base having the coating layer formed thereon: impregnating thefriction base with a phenol resin; squeezing the friction baseimpregnated with the phenol resin; and performing heat treatment forhardening the phenol resin incorporated into the friction base.